In a next generation network (NGN), packet techniques such as IP are adopted as a bearer network technique to integrate fixed communications and mobile communications. The NGN network is an integrated network that makes use of IP connectivity among telecommunication entities to carry various types of telecommunication services such as voice, image, and data and so on. Although the deployment of NGN has already begun, such problems as quality of service (QoS), security, reliability, operability and manageability of the IP bearer network still need a perfect solution.
Two international standard organizations, ITU-T and ETSI, have respectively set up a special research group dedicated to draft the standards for NGN technique, which are in an architecture stage at present. It has become an agreement of a majority of main operators and venders to use a resource and admission control subsystem (RACS) to solve the problems of QoS, network address translation (NAT), and firewall traversal within the NGN bearer network.
The scopes of the RACS drafts of the two organizations are different, e.g. the ITU-T FGNGN considers the packet transport parts of core networks besides the packet transport parts of access networks; but the architectures of the two organizations are similar and compatible. Both of them make reference to an idea of admission control based on operation policy rules at edges of the network, which is used by 3GPP PDF (3rd generation partner project policy decision function) and PacketCable GC (Gate Controller). The result of checking transport resource availability within networks is added to both RACS drafts to serve as one of the conditions of admission control, so that unacceptable congestion, delay or packet loss for the application layer, which may result from that service traffic and their QoS class requirements exceed the network bearer capabilities, are avoided. At the same time, NAT, firewall traversal and support of various NGN multimedia services are taken into consideration in both RACS drafts.
The architecture definition in the current RACS drafts has been substantially stable and the RACS functional architecture needs to support various types of resources reservation mechanisms. At present, there are the following three resources reservation modes.
(1) Proxy-requested Mode                Service control equipment initiates a resources request to the resource and admission control layer on behalf of the user. The service control equipment can extract the explicit bandwidth and QoS requirements of the media flows of the session which are contained in the user service request signaling, or determine the bandwidth and QoS requirements of the media flows of the session according to the operation policy rules (for example, service type or encoding/decoding type) of the service layer.        
(2) CPE-requested Mode                Customer premises equipment (CPE) initiates a resources request to the network through a dedicated path coupling signaling. The resources request can be initiated for each session or be initiated concerning no session. The resources request can either be authorized in advance by the service control device or not.        
(3) Access Configuration Mode                When a subscriber accesses the network, network access management equipment initiates a resources request to the network according to subscriber service level agreement (SLA) information in the subscriber configuration file. The resource request is in per subscriber-level and has no association with the sessions.        
The structure of the functional architecture of RACS is provided in the latest RACS draft which is released by the ITU-T FGNGN in December, 2004, as shown in FIG. 1.
An application function entity (AF) refers to a service layer function entity which requests for resource and admission control and requests the resources reservation and release for application media flows; the AF includes such function entities as a session control proxy function (SCPF), an interconnection border control function (IBCF) and the like.
An network access attachment function (NAAF) refers to functions which are in charge of network access management control; the NAAF includes network access authentication, authorization and dynamic IP address allocation of subscriber equipment, configuration for access equipment, storage of subscriber configuration files and the like.
A Resources mediation entity (RM) receives the resources request, checks whether the resources requests are consistent with the operation policy rules, interacts with related functions to check whether the resources requests are consistent with the subscriber configuration information stored in a subscriber configuration database and whether there are idle and available transport resources within networks meeting the resources reservation requirements, and makes an admission decision according to the above checking results. The resources mediation entity includes a mediation policy decision function entity (M-PDF) located in the service access side and an interconnection policy decision function entity (I-PDF) located in the service interconnection side.
A transport resources control function entity (TRCF) receives a resources request from the resources mediation functions, checks whether there are idle and available transport resources within networks for the resources request according to the collected and maintained network topology and resources status database, and controls the QoS related forwarding actions of transport equipment in networks. If there are multiple transport resources control function entities in one network, these entities communicate with each other to check the availability of transport resources from the ingress edge to the egress edge within the whole network. A transport resources control function entity includes a transport resources control function entity located in the access network (A-TRCF) and a transport resources control function entity located in the core network (C-TRCF).
A border gateway function entity (BGF) of the core network receives, stores and executes information of admission decision parameters received from the resource and admission control layer, performs packet filtering, traffic classification, marking, policing and shaping, and may further perform network address translation and security inspect filtering on packets.
The functional architecture of RACS is also provided in the latest RACS draft which is released by the ETSI TISPAN in January, 2005, as shown in FIG. 2.
An application function (AF) refers to a service layer function entity which requests for the resource and admission control and requests the resources reservation and releasing for an application media flow.
A network additional subsystem function (NASS) refers to functions which are in charge of network access management control; the NASS includes network access authentication, authorization and dynamic IP address allocation for subscriber equipment, configuration for access equipment, storage of subscriber configuration files and the like.
A service-based policy decision function (SPDF) receives a resources request, checks whether the resources request are consistent with the operation policy rules, interacts with an access network resources admission and control function (A-RACF) to check whether there are idle and available transport resources in access networks meeting the requirements, and then makes an admission decision according to the above checking results.
The A-RACF receives a resources request from the SPDF, detects whether there are idle and available transport resources in the access network meeting the requirements, and controls the QoS-related forwarding actions of transport equipment in networks for assuring the QoS.
A border gateway function entity of the core network (C-BGF) receives, stores and executes information of admission decision parameters received from the resource and admission control layer, performs packet filtering, traffic classification, marking, policing and shaping, and may perform network address translation and security inspect filtering on packets.